1. Field of the Invention
This invention relates to an ignition plug insulator provided at its front end with a closed ignition portion.
2. Description of the Prior Art
In general, an ignition plug functions as a part of an electric ignition circuit to produce an electric spark between a center electrode and an outside electrode such that a high voltage discharge is guided into a combustion chamber so as to reliably ignite a mixed fuel gas and effect combustion. As a result, the ignition plug is exposed to high temperature and high pressure due to explosion of the mixed fuel gas and to sudden drop of the temperature and pressure due to suction of a new mixed fuel gas. The ignition plug insulator, therefore, is required to have a strength which is sufficient to not only withstand the high electric voltage but also withstand thermal shock and mechanical oscillation and shock. In addition, the ignition plug insulator is required to have a resistant property against chemical and thermal actions subjected thereto due to combustion gas and high temperature flame.
Purification of the exhaust gas from automobiles has recently become a very important matter. Automobiles incorporating apparatus for purifying such exhaust gas are now available in market. Meanwhile, the spread of frequency modulation radio receivers and mobile radios has raised another problem of eliminating noises due to electric waves emitted mainly from an ignition system of the automobile. It is desirous, therefore, to provide measures of eliminating these difficulties.
In order to eliminate both the public pollution due to exhaust gas and the electric wave disturbance, it is quite important to improve the ignition system, particularly its ignition source, that is, an ignition plug. All possible efforts are now exerted to study and develop the ignition plug.
The cruxes of the problem are as follows.
1. Alleviation of flame extinguishing action of a center electrode of an ignition plug. PA1 2. Suppression of noise current produced during spark discharge of the ignition plug. PA1 1. The electric conductivity imparting material is formed of at least one metal selected from the group consisting of Cu, Fe, Co, Mn, Cr, Ti and La, alloys, oxides or oxide semiconductors thereof. PA1 2. The front end of the ignition portion of the hollow porcelain insulator has a thickness within a range between 0.2 mm and 2.0 mm. PA1 3. The ceramic center electrode has a resistance value of at most 10 M.OMEGA..
The flame extinguishing action of the center electrode results in a so-called miss firing of the ignition plug. As a result, as a concrete means for decreasing the miss firing, it has been the common practice to widen an electrode gap, for example, widen the usual electrode gap of 0.7 to 0.9 mm up to the order of 1.1 to 1.5 mm. In practice, such countermeasure can fairly improve the ignition ability of the spark plug.
However, the spark voltage across the electrodes becomes higher substantially in proportion to the electrode gap, and as a result, the above mentioned means for widening the electrode gap has the disadvantage that the ignition system must be to considerably changed in design by including a special transistor ignition source, for example, thereby increasing the manufacturing cost.
It has also been proposed to cover the front end of the center electrode with ceramic or the other refractory material for the purpose of alleviating the flame extinguishing action of the center electrode without increasing the discharge voltage and hence of improving the ignition property of the ignition plug. Such means, however, does not always bring satisfactory results owing to considerable difference between the thermal expansion coefficient of the covering material and that of the metal of the front end of the center electrode.
On the other hand, a so-called resistance incorporated plug provided in an axial bore of a procelain insulator with a resistor inserted or formed therein has been proposed for the purpose of suppressing the noise electric wave produced during the spark discharge of the ignition plug. Such resistance incorporated plug can effectively suppress generation of the noise electric wave and has widely been used in U.S.A. The demand for such resistance incorporated plug is also increasing in Japan. Particularly, in Canada where the electric wave regulation has really been adopted since September 1977, it is required to provide a resistance incorporated plug which can effectively prevent noise up to a high frequency band on the order of 1,000 MHz.